The measurement of the total organic carbon (TOC) concentration and total carbon (organic plus inorganic) concentration in water has become a standard method for accessing the level of contamination of organic compounds in potable waters, industrial process waters, and municipal and industrial waste waters. In addition to widespread terrestrial applications, the measurement of TOC is one of the primary means of determining the purity of potable and process waters for manned space based systems including the space shuttle, the proposed space station and for future manned explorations of the moon and planets.
The United States Environmental Protection Agency recently promulgated new rules aimed at reducing the levels of disinfectant by-products in drinking water. Formed from the reaction of chlorine and other disinfectants with naturally occurring organic matter, disinfectant by-products are potentially hazardous compounds including trihalomethanes (CHCl.sub.3, CHBrCl.sub.2, etc.), haloacetic acids, and other halogenated species. The new rules also include monitoring the levels of natural organic material in raw water, during the treatment process and in the finished water by measurement of total organic carbon concentration.
A variety of prior art approaches for measuring the total organic carbon content of water have been proposed. For example, See U.S. Pat. Nos. 3,958,941 of Regan; 3,224,837 of Moyat; 4,293,522 of Winkler; 4,277,438 of Ejzak; 4,626,413 and 4,666,860 of Blades et al.; and 4,619,902 of Bernard.
Representative of the devices described in these references are the methods disclosed in U.S. Pat. No. 3,958,941 of Regan. In Regan an aqueous sample is introduced into a circulating water stream that flows through a reaction chamber where the sample is mixed with air and exposed to ultraviolet (U.V.) radiation to promote the oxidation of organic compounds to form carbon dioxide. The carbon dioxide formed in the reaction chamber is then removed from solution by an air stripping system and introduced into a second chamber containing water that has been purified to remove ionic compounds. The conductivity of the water in the second chamber is measured, and any increase in conductivity is related to the concentration of carbon dioxide formed in the first reactor. The conduction measurement can be used, therefore, to determine the concentration of organic compounds in the original sample.
The Regan device is slow, cannot be used for the continuous monitoring of TOC concentration in aqueous streams, cannot be scaled down without increasing interference from NO.sub.2, CO.sub.2, and H.sub.2 S to unacceptable levels, and is generally unsatisfactory. In addition, Regan does not disclose that an aqueous solution of acid must be added to the sample stream to reduce the pH to a value of less than about 4 to ensure a reasonable removal rate of carbon dioxide using the air stripping system described. The oxidation method disclosed by Regan is unsatisfactory for the measurement of refractory compounds, particularly urea. In Regan, an aqueous sample of 20 to 100 mL containing 0.5 mg/L organic carbon is required to generate sufficient carbon dioxide for accurate detection, thus limiting the utility of the device for the measurement of sub-part per million levels of TOC in smaller sample sizes. Finally, in practice, the Regan system requires frequent recalibration--typically once per day--due to variations in background conductivity. Also, the concentration of total organic carbon in the calibration standard must be approximately equal to the concentration of organic carbon in the sample. Because of this, recalibration is required when analyzing aqueous samples containing higher or lower levels of organic carbon when compared with the calibration standard.
The use of aqueous solutions of persulfate salts for the oxidation of organic compounds is widely known. Smit and Hoogland (16 Electrochima Acta, 1-18 (1971)) demonstrate that persulfate ions and other oxidizing agents can be electrochemically generated. In U.S. Pat. No. 4,504,373 of Mani et al., a method for the electrochemical generation of acid and base from aqueous salt solutions is disclosed.
An improved method and apparatus for the measurement of organic content of aqueous samples is disclosed in U.S. Pat. No. 4,277,438 of Ejzak. Ejzak describes a multistage reactor design which provides for the addition of oxygen and a chemical oxidizing agent, preferably sodium persulfate, to the aqueous sample stream prior to oxidation of the stream using ultraviolet radiation in a series of reactors. Ejzak also describes the use of an inorganic carbon stripping process--before oxidation of the organic carbon--that includes the addition of phosphoric acid to the sample stream. After oxidation, the sample stream is passed into a gas-liquid separator where the added oxygen acts as a carrier gas to strip carbon dioxide and other gases from the aqueous solution. In the preferred embodiment, the gas stream is then passed through an acid mist eliminator, a coalescer and salt collector, and through a particle filter prior to passage into an infrared (IR) detector for the measurement of the concentration of carbon dioxide in the gas stream.
The methods and apparatus disclosed by Ejzak provide improvements over the teachings of Regan; however, the Ejzak device requires extensive manual operation and is also generally unsatisfactory. The Ejzak device requires three external chemical reagents; oxygen gas, aqueous phosphoric acid and an aqueous solution of sodium persulfate. Both the phosphoric acid and persulfate solutions must be prepared at frequent intervals by the operator due to the relatively high rate of consumption. The Ejzak device requires dilution of the sample if the solution contains high concentrations of salts in order to ensure complete oxidation of the sample and to eliminate fouling of the particle filter located prior to the IR carbon dioxide detector. As with Regan, relatively large sample sizes are required--typically 20 mL of sample for accurate measurement at 0.5 mg/L total organic carbon--and the carbon dioxide formed in the oxidation chamber is removed using a gravity dependent technique that cannot be easily used in space-based operations.
Another improved method and apparatus for the measurement of total organic carbon in water is disclosed in U.S. Pat. No. 4,293,522 of Winkler. In Winkler, an oxidizing agent, molecular oxygen, is generated in-situ by the electrolysis of water. Organic compounds are subsequently oxidized to form carbon dioxide by the combination of U.V. radiation and the in-situ generated oxygen. The irradiation and electrolysis processes are both accomplished in a single oxidation chamber. Winkler does not teach that the aqueous sample stream be acidified to assist in the removal of carbon dioxide from solution, and in fact teaches against the use of acid. Therefore, this method and apparatus cannot be used for the measurement of organic compounds in basic aqueous samples. The oxidation chamber of Winkler uses a solid electrolyte to separate the two electrodes employed for the electrolysis of water. The solid electrolyte described by Winkler is composed of an organic polymer which, under exposure to oxygen, ozone and U.V. radiation, will undergo oxidation to form carbon dioxide, therefore resulting in unacceptable background levels of organic compounds in the sample stream, particularly at low organic compound concentrations.
Winkler also describes a conductometric carbon dioxide detection system wherein the sample stream exiting the oxidizing chamber is held in an equilibrating relationship to a stream of deionized water. The two flowing streams are separated by a gas permeable membrane that allows the concentration of carbon dioxide to equilibrate between the streams. The concentration of the carbon dioxide is thereby determined by measuring the conductance of the deionized water stream. However, the use of two flowing streams introduces operating parameters into the detection process that require frequent calibration adjustments.
Another example of the prior art is disclosed in U.S. Pat. No. 4,619,902 of Bernard, which teaches the oxidation of organic compounds to form carbon dioxide using persulfate oxidation at elevated temperatures--typically 20 to 100.degree. C.--in the presence of a platinum metal catalyst. Bernard recognizes that the materials used in the construction of instrumentation for the determination of total organic carbon in water can contribute organic compounds to the sample during the measurement process, and teaches that inert materials such as PTFE must be used to reduce this background from the measurement. As with the previously mentioned disclosures, a gas stripping technique is employed to collect the formed carbon dioxide, and measurement is made using IR spectrometry. Bernard also recognizes that aqueous solutions of sodium persulfate are not stable due to auto-degradation of the reagent.
An improved system for the measurement of organic compounds in deionized water is disclosed in U.S. Pat. No. 4,626,413 of Blades and Godec. The apparatus described by Blades and Godec is based on direct U.V. oxidation of organic compounds to form carbon dioxide which is measured by using conductometric detection. In the apparatus described in Blades and Godec, the oxidation of some organic compounds form strong acids such as HCl, H.sub.2 SO.sub.4 and HNO.sub.3 which interfere with the conductometric method. The Blades device is also limited to the measurement of total organic compounds in deionized water and cannot be used for samples containing ionic compounds other than bicarbonate ion.
In U.S. Pat. No. 4,209,299 of Carlson, it is disclosed that the concentration of volatile materials in a liquid can be quantitively determined by transferring the desired material through a gas permeable membrane into a liquid of known conductivity, such as deionized water. The Carlson device is demonstrated for the measurement of a number of volatile organic and inorganic compounds, but Carlson does not suggest the combination of this process in conjunction with a carbon dioxide producing reactor.
In electrochemical reactions in aqueous solutions, a common reduction product is hydrogen gas. Because of its flammability, the hydrogen presents a potential hazard in devices using electrochemical techniques. The interaction of hydrogen gas in aqueous solutions and palladium metal is well known (e.g., F. A. Lewis, "The Palladium Hydrogen System," Academic Press, 1967, London, incorporated herein by this reference) and the use of palladium offers a potential solution to the generation of hydrogen in electrochemical reactions by selective removal and disposal of the hydrogen.
An improved carbon analyzer is disclosed in U.S. Pat. No. 5,132,094 by Godec et al., of which the present is a continuation-in-part. Originally developed for NASA, the Godec device uses UV/persulfate oxidation and a new CO.sub.2 detection technique and membrane-based conductivity. A gas-permeable membrane is used to separate the acidified sample stream (pH&lt;2) from a thin layer of deionized water. A solenoid valve is opened to allow fresh DI water to flow into the membrane module and the solenoid valve is closed. Carbon dioxide formed from the oxidation of organic compounds will diffuse across the membrane into the deionized water, where at a pH of about 7 a portion of the CO.sub.2 will ionize to produce H+ and HCO.sub.3 -ions. After an equilibration period, the solenoid valve is opened to flush the ions into a conductivity and temperature measurement cell, and the concentration of CO.sub.2 in the deionized water is determined from the conductivity.
Membrane-based conductivity detection of CO.sub.2 offers several advantages. Calibration is extremely stable, and the calibration can be easily performed by the analyst. No purge gases are required. The technique is highly selective for CO.sub.2 and is extremely sensitive, permitting detection of TOC down to sub-parts per billion levels. It also has a wide dynamic range, permitting measurement up to 50 ppm TOC.
In operation the sample is drawn into the analyzer by means of a peristaltic pump, and two reagents are added via syringe pumps. Acid (6 M H.sub.3 PO.sub.4) is added to reduce the pH of the sample stream and persulfate (15% (NH.sub.4).sub.2 S.sub.2 O.sub.8) is added for the oxidation of organic compounds. The sample stream is split for measurement of inorganic carbon (IC) concentration (IC=HCO.sub.3 --!+CO.sub.3 --.sup.2 !) without oxidation, and measurement of total carbon (TC) concentration after oxidation. TOC is then computed from the difference (TOC=TC-IC). For samples containing high levels of inorganic carbon and lower levels of TOC, an IC removal module may be used to remove the inorganic carbon and permit accurate TOC measurements. A supply of the acid and oxidizer may be pre-packaged and stored in the analyzer, eliminating the need for reagent preparation by the analyst. Deionized water is continuously produced in the analyzer using a mixed-bed ion exchange resin with a capacity for several years of operation. The maintenance required is replacement of the reagent containers several times a year, replacement of the UV lamp and replacement of the pump tubing. The ease of use, low maintenance requirements and dependable performance has made this device the TOC analyzer of choice for monitoring water purification systems in semiconductor manufacturing, the pharmaceutical industry and conventional and nuclear power plants.
It is important that the amount of persulfate or other oxidizer added to the sample be sufficient to fully oxidize the sample. However, it is also important not to add excess oxidizer to the point that gas bubbles form in the sample. Gas bubbles are undesirable because the carbon dioxide dissolved in the sample will diffuse into the oxygen bubbles. If the oxygen bubble diffuses through the membrane and into the deionized water stream, the result will be a negative spike in the measured conductivity as the bubble passes through the conductivity cell due to the changed flow volume.
This has been addressed in the past by controlling the addition of oxidizer based on the expected approximate range of carbon concentration. For example, the oxidizer flow rate would be set relatively low if the expected carbon concentration were in the 1 to 5 ppm range, and the oxidizer flow rate would be set higher if the expected carbon concentration were in the 25 to 50 ppm range. This is a very effective approach. However, it would be desirable for the device to produce accurate readings across a broad range of carbon concentrations with a minimum of experimentation or prior knowledge about the approximate expected carbon concentrations.
It has also been found in utilizing prior devices that chloride in the sample tends to lead to inaccurate measurements of carbon concentrations, because the chloride preferentially interacts with hydroxyl radicals to the exclusion of organics, thus exhausting the oxidizer before the organics are fully oxidized.